1. Field of the Invention
This invention is based on the earlier invention of the seismometer group recorder, which is a field system for recording seismic signals at a plurality of geophones in a spread without the need for connecting the individual geophones through cables to a recording truck. Each of the group recorders is a small portable recording unit placed near the center of the seismometer group location and operated such that upon specific "call" from the communications link of a central control unit, a particular designated one or set of the group recorders present in the field are turned on and are thus ready to record as soon as the seismic source is actuated. However, the operating people need to have a signal perceptible from 20 to 100 feet away or more, that the proper designated recorders are now functional.
2. Description of the Prior Art
It is well known to have some sort of a visual or aural indication when a piece of electronic equipment is energized or switched on, such as a glowing pilot bulb. However, in the field of seismic prospecting, it has not been considered important in the past to have more than a single pilot light per amplifier, which did no more than indicate that that particular amplifier had been energized.
On the other hand, upon the invention of the seismometer group recorder (the subject matter of U.S. Pat. No. 3,806,864), there was need for a quite different system which will tell the field forces using the equipment the status of the recorders selected out of the total by the use of the transmission or communications link. It should be recalled that in this system there are a large number of geophones connected in groups to recorders scattered along the spread, and that ordinarily not over about half the group recorders are switched on before a shot or other source produces the seismic waves which are recorded, by the communications link (frequently a radio). This sent out very rapidly a set of coded signals each of which turned on an individual group recorder. Now it is necessary not only to be assured of the fact that the battery in the device has been switched on, but that there are geophones connected to the group recorder, a tape cassette has been inserted, the tape in the cassette has not been used up, the battery voltage is above a specified minimum, etc. When it is recalled that the cost of the data obtained on the cassette tapes is large and that frequently the data is literally nonreplaceable, it can be seen that there is a very strong interest in assuring that the equipment is in good working order immediately before a seismic record is made. Furthermore, the various group recorders may be scattered up and down a line well over a mile in length, so that it is not practical to have enough field men required to visit each station in turn and notice if the recorder is ready for operations. Having an aural signal or a flashing light at each recorder indicating that all is ready immediately prior to recording makes this a much easier task.